1. Field of the Invention
This invention relates to a high-impedance substrate that employs an artificial medium.
2. Description of the Related Art
With a view to reducing the electric power of the antenna for handling the radio wave of a high-frequency region, there have been proposed various methods. For example, there has been proposed a method wherein a unit particle made of a metal and having a size which is almost the same as or smaller than the wavelength of an electromagnetic wave to be employed is used and, at the same time, the manner of arranging the unit particle is devised. Using this method, it has been made possible to realize an artificial medium having characteristics which differ from the physical properties which the material inherently has and to apply the artificial medium to a left-handed system medium, a resonator and artificial dielectric.
Further, there has been proposed a technique to enhance the characteristics of an antenna through the utilization of the phenomenon that an artificial medium having resonators arranged periodically is capable of acting, while achieving high-impedance, at a frequency in the vicinity of a band gap. This technique, however, is accompanied with the problem that when the capacitance “C” is increased, the normalized bandwidth becomes smaller.
On the other hand, there is an advantage that when the inductance “L” is increased, the normalized bandwidth can be made larger and it is possible to lower the frequency of the radio wave. Although there is known a method of increasing the thickness of the antenna for the purpose of increasing the inductance “L”, this may conflict with the demand to realize a thinner substrate. Under the circumstances, it is desired to increase the inductance “L” through the increase of magnetic permeability “μ” with a magnetic material.
For example, JP-A 2005-538629 (KOHYO) discloses a high-impedance substrate having a mushroom structure using ferrite as a magnetic material. The magnetic materials employed in this publication are, in most cases, not only large in magnetic permeability but also large in dielectric constant, thus resulting in an increase of the capacitance “C”. As a result, the normalized bandwidth becomes smaller. Namely, up to the present, no one has succeeded to obtain a thin high-impedance substrate having a large normalized bandwidth at a low frequency band.